1. Field of the Invention
The present invention relates to a magnetic storage medium suitable for a magnetic disk unit for performing recording and regeneration of information.
2. Description of the Related Art
As recording information, which is dealt with in an information processing apparatus, is increased, it is needed to provide magnetic storage, which is used as an external storage unit of the information processing apparatus, with a compactness and the larger capacity. For this reason, the magnetic storage needs a magnetic storage medium capable of recording at high recording density. However, according to the conventional magnetic storage medium, it is known that as magnetic information is recorded at higher recording density, S/Nm of the recorded magnetic information to the reproductive signal is lowered (the medium noise Nm is increased with respect to the output S of the reproductive signal).
Generally, a magnetic storage medium has a recording layer on which magnetic information is recorded. One bit of magnetic information is represented by a direction of a total magnetization consisting of an assembly of the respective magnetizations of a plurality of ferromagnetic crystal particles existing in a one bit cell of the recording layer. It may be considered that the magnetizations in the one bit cell are substantially unified in direction in the state that magnetic information is recorded. However, in the event that the magnetization of the adjacent one bit cell is unified in a direction opposite to that of the noticed one bit cell, the direction of the magnetization is reversed through a certain width near a boundary between the adjacent one bit cell-to-one bit cell, but not rapidly changed on the boundary. In an area having such a width, magnetizations oriented in mutually opposite directions are mixed on a zigzag basis. This area is referred to as a magnetization transitional region. One of the causes of the occurrence of the medium noise mentioned above resides in the unevenness of the magnetization.
It is known that the unevenness of the magnetization is caused by a magnetic interaction between crystal particles offering the ferromagnetism. The inventors of the present application proposed a granular magnetic storage medium as a medium in which the magnetic interaction as mentioned above is weakened (Japanese Patent Application Hei. 07-160437). The proposed granular magnetic storage medium has a recording layer in which a plurality of ferromagnetic crystal particles, which consist of an Fe or Fe system-alloy or a Co or Co system-alloy, are dispersed in a non-magnetic material which is of a non-solid solution to those alloys, and are mutually isolated. Thus, the mutual isolation between the plurality of ferromagnetic crystal particles may substantially completely divide the magnetic interaction between the ferromagnetic crystal particles into parts, and thereby reduce the medium noise due to an unevenness of the magnetization in the magnetization transitional region.
The medium noise occurs also owing to an unevenness of a particle size of the ferromagnetic crystal particles. It is considered that the regenerative output is in proportion to the sum total of the volume of ferromagnetic crystal particles. Hence, as the average particle size of one bit cell becomes large, unevenness of a particle size becomes also large. As a result, unevenness of the regenerative output becomes large and thus the medium noise is increased. Therefore, it is considered that the medium noise Nm of the magnetic storage medium is decreased in such a manner that the particle size of the ferromagnetic crystal particles is controlled in the magnetic recording layer of the magnetic storage medium, so that S/Nm is improved.
However, with respect to the magnetization recorded on the ferromagnetic crystal particles in which the magnetic interaction is divided into parts and magnetically isolated, as the particle size of the ferromagnetic crystal particles is decreased, the energy Kuxc2x7V (anisotropy energy x volume of particle) representative of a degree of easy orientation of magnetization of the particle into a predetermined direction is reduced. When the energy Kuxc2x7V is reduced, a thermal fluctuation phenomenon wherein a direction of magnetization fluctuates owing to the heat will occur. Consequently, in a case where the particle size of the ferromagnetic crystal particles is less than a predetermined size, there occurs, even in the room temperature, the thermal fluctuation phenomenon in magnetization of each of the ferromagnetic crystal particles. This is associated with a problem that the recording magnetization on a one bit cell consisting of the total sum of pieces of magnetization of the ferromagnetic crystal particles disappears.
In order to maintain the energy Kuxc2x7V with a large value while the volume of ferromagnetic crystal particles is reduced, it is considered that one having the large anisotropy energy Ku is adopted as the material of the ferromagnetic crystal particles. However, with a conventional recording head now generally used, it is difficult to generate such a strong magnetic field that the magnetization of the crystal particles having such a large anisotropy energy Ku is reversed. This is associated with a problem that an overwrite characteristic (O/W) of a magnetic storage medium, which is represented by a ratio of magnitudes of a reproductive output of magnetic information remained at the time of re-recording of magnetic information and a regenerative output of magnetic information newly recorded, is deteriorated.
In view of the foregoing, it is an object of the present invention to provide a magnetic storage medium capable of recording information at high recording density and also to regenerating the information with a high quality of signal (high S/Nm), and in addition contributing to the elongation of a life span of the recorded information.
To achieve the above-mentioned objects, the present invention provides a magnetic storage medium comprising:
(1) a non-magnetic substrate;
(2) a recording layer in which a plurality of crystal grains consisting of a ferromagnetic material are dispersed in a non-magnetic material and axes of easy magnetization of the plurality of crystal grains are oriented in parallel to the layer,
wherein each of the plurality of crystal grains dispersed in the recording layer is a columnar shaped grain which penetrates a surface of said recording layer and has height of 30 nm or less, and a mean value of grains diameter of said recording layer is 15 nm or less.
Furthermore, (Kuxc2x7V)/(kBxc2x7T) is preferably not less than 60 where (Kuxc2x7V)/(kBxc2x7T) denotes a ratio of a product of a mean value Ku (unit: erg/cm3) of anisotropy energies of the plurality of crystal grains and a mean value V (unit: cm3) of volumes of the plurality of crystal grains to a product of Boltzmann constant kB (unit: erg/K) and temperature T (unit: K), and a mean value HK of anisotropic magnetic fields of the plurality of crystal grains is 20 kOe or less.
As a magnetic storage medium of the next generation, a magnetic storage medium having a high recording density exceeding 10G bit/inch2 is desired. A size of one bit cell of the magnetic storage medium having a recording density exceeding 10G bit/inch2 is 100 nmxc3x97600 nm or so.
In the event that such a large size of one bit cell is provided, a magnetic storage medium of the present invention has characteristics set forth below.
First, according to the magnetic storage medium of the present invention, a mean value of grain sizes of the plurality of crystal grains in an internal direction of the recording layer is kept below 15 nm. This feature makes"" it possible that the magnetic storage medium offers a high S/Nm not less than 25 dB in which magnetic information to be stored is reproduced with great accuracy. Further, in the event that the plurality of crystal grains have such a grain size, the thickness of the recording layer is kept below 30 nm. This feature makes it possible to keep the width of the magnetization transitional region below a predetermined value, and thereby enhancing a resolution which is an index indicative of a recording density involved in an effective recording. Furthermore, according to the magnetic storage medium of the present invention, (Kuxc2x7V)/(kBxc2x7T) is not less than 60. This feature makes it possible to prevent the thermal fluctuation of magnetization of a plurality of crystal grains of the recording layer whereby magnetic information to be stored in the recording layer is ensured a longer life, necessary for practical usage. Still further, according to the magnetic storage medium of the present invention, the mean value HK of anisotropic magnetic fields of the plurality of crystal grains is 20 kOe or less. This feature makes it possible to offer a satisfactory O/W.
In order to obtain a value of O/W requested for a practical use through a further improvement of O/W, it is effective that the mean value HK is further decreased. It should be noted that generally, the mean value Hu (unit: erg/cm3) of anisotropy energies of the crystal grains is decreased while the mean value HK of anisotropic magnetic fields of the plurality of crystal grains is decreased. Consequently, too much reduction of the mean value HK brings about reduction of the mean value Ku. As a result, (Huxc2x7V)/(kBxc2x7T) goes down below 60 (limit value). However, according to the magnetic storage medium of the present invention, each of the crystal grains dispersed in the recording layer is a columnar shaped grain which penetrates a surface of the recording layer. Hence, according to the magnetic storage medium of the present invention, as compared with a case where the spherical crystal grains are imbedded, the crystal grains are advantageous in volume in a height direction, and thereby increasing the mean value V of volumes of the crystal grains. The increment of the mean value V of volumes of crystal grains may maintain the value of (Huxc2x7V)/(kBxc2x7T), while permitting the reduction of HK and the reduction of HK as well.
Further, even if the same HK is concerned, according to the magnetic storage medium of the present invention, each of the crystal grains of the recording layer penetrates the recording layer, and therefore the magnetization of the crystal grains is oriented in an internal direction of the recording layer on a priority basis. The orientation of the magnetization of the crystal grains is easy to reverse using a magnetic field of the head when magnetic information is being rewritten, and thus the O/W of the magnetic storage medium is further improved.
As mentioned above, the magnetic storage medium according to the present invention has a high resolution and a high S/Nm as well, and can store magnetic information having a long life necessary for practical usage, and offers a high O/W in a practical use of level.
In the magnetic storage medium according to the present invention as mentioned above, it is preferable that the substrate is a disk-like shaped substrate, and the axes of easy magnetization of the crystal grains included in the recording layer are oriented to a circumference direction of the disk-like shaped substrate.
Generally, a magnetic storage medium having a high recording density is formed on a disk-like shaped substrate, and a direction of a magnetic field of a head for recording and reproducting magnetic information on the magnetic storage medium is oriented to a circumference direction of the disk-like shaped substrate. When the axes of easy magnetization of the plurality of crystal grains included in the recording layer are oriented to a circumference direction of the disk-like shaped substrate, the magnetic storage medium is improved in O/W. Further, this orientation causes the coercive force Hc of the medium in a circumference direction to be increased, so that a width of the magnetization transitional region is narrowed. Thus, the magnetic storage medium is enhanced in the resolution.
Further, in the magnetic storage medium according to the present invention as mentioned above, it is preferable that the crystal grains of said recording layer consist of an alloy including Co and Pt.
Co is a ferromagnetic material having a hexagonal crystal structure and an uniaxial crystal magnetic anisotropy, and is suitable for a material of a recording layer since the magnetization it easy to be oriented. The addition of Pt to Co makes it possible to enhance a coercive force Hc of the recording layer. Thus, the magnetic storage medium having the crystal grains consisting of an alloy including those elements is improved in the resolution.
Further, in the magnetic storage medium according to the present invention as mentioned above, it is preferable that the non-magnetic material of the recording layer consists of an oxide ceramics.
Since the oxide ceramics is of a non-solid solution to a metal, according to the magnetic storage medium having such a structure, it is possible to expect a preferable separation between the crystal grains. Thus, the medium noise Nm of the magnetic storage medium is decreased, and S/Nm is improved.
Further, in the magnetic storage medium according to the present invention as mentioned above, it is preferable that
(3) a primary layer consisting of a non-magnetic material is interposed between the substrate and the recording layer.
The crystal grains are subjected to a hetero-epitaxial growth on the interface of the upper portion of the primary layer. Therefore, the magnetization of the crystal grains is increased in orientation.
Further, in the magnetic storage medium according to the present invention as mentioned above, it is preferable that said primary layer consists of an alloy including Cr.
A face interval of the (110) of Cr is close to a face interval of the (002) of the CoPt-alloy of the crystal grains. Thus, when the crystal grains are subjected to a hetero-epitaxial growth on the interface of the upper portion of the primary layer, the magnetization of the crystal grains is oriented in an internal direction of the recording layer on a priority basis.
Further, in the magnetic storage medium having the primary layer (3) as mentioned above, it is preferable that an intermediate layer consisting of a non-magnetic material, which is different from the material constituting the primary layer, is interposed between the primary layer and the recording layer.
The intermediate layer is disposed between the primary layer and the recording layer. The use of the intermediate layer makes it possible to reduce an unevenness of the grain sizes of a plurality of crystal grains included in the recording layer. Consequently, the magnetic storage medium having the intermediate layer offers a high S/Nm.
In the magnetic storage medium having the intermediate layer as mentioned above, it is preferable that the primary layer consists of an alloy including Cr, and the crystal grains of the recording layer consist of an alloy including Co and Pt, and in addition the intermediate layer consists of an alloy including Co and Cr.
The alloy including Co and Cr is an intermediate material between an alloy including Co and Pt of the recording layer and an alloy including Cr of the primary layer, and thus suitable as a material of the intermediate layer between the recording layer and the primary layer. Further, the crystal grains of the recording layer can be subjected to a hetero-epitaxial growth on the intermediate layer.
Further, in the magnetic storage medium having the primary layer (3) as mentioned above, it is preferable that a seed layer consisting of a material, which is different from the material constituting the primary layer, is interposed between the substrate and the primary layer.
A surface of the substrate is sparse. Interposing the seed layer between the substrate and the primary layer provides a smoothness of a surface of the primary layer. The smoothness of the primary layer makes it possible to improve the crystal grains of the recording layer in an orientation of the magnetization.